If the outside of the kilt is towards the viewer then in your diagram the seam is actually to the right of B, and it involves the outer fold of the pleat, in your diagram it is to the right, not the inner fold of the previous pleat.

There are two different methods of doing the tapering.

One is done on wool and wool mix kilts, because they are more pliable. If you fold the fabric to form the inner crease and sew through the two layers - you can do that on a sewing machine, you slant the line of the sewing so as to make the distance from the fold decrease as it gets further from the waistline. As it is a slant the seam is longer than the straight grain and will need to be pressed to fit.

The other is more usually seen on cotton or other rigid fabrics. There the outer edge of the pleat is kept straight on the grain of the fabric, but it is sewn down onto the fabric beneath it a slant.

When using a sewing machine you normally can't pin in all the pleats, but do one at a time so you can get at it easily.

If you take a piece of fabric and fold it up you should see how the pleats overlap and would get in the way of machine sewing if they were all pinned down.